1. Field of the Invention
This invention relates to cellular telephone network data transmission, specifically to a method of scheduling packet data transmission for a connection-less packet service.
2. Prior Art
Packet data communication is known in cellular telephone systems, as is evidenced by, for example, commonly assigned U.S. Pat. No.: 5,257,257, issued Oct. 26, 1993, entitled xe2x80x9cMethod of Controlling the Operation of a Packet Switched CDMA Communication Network for Controlling the Operation of Transmitters and Receiversxe2x80x9d, by X. H. Chen and J. Oksman.
One further example is defined in TIA/EIA/IS-657, Packet Data Service Option for Wideband Spread Spectrum Cellular System. IS-657 is used along with TIA/EIA/IS-95A, Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, and, TIA/EIA/IS-99 Data Services Option Standard for Wideband Spread Spectrum Digital Cellular System. The current IS-657 based packet data scheme for code division multiple access (CDMA) does not allow the traffic channel to be shared between more than one user. The IS-657 scheme is based on a make-and-break of multiple traffic channel connections during the life of a packet data session. In the worst case, a packet may suffer a call set-up delay which can range from tens of milliseconds to a few seconds. Also, it is possible for a mobile station (MS) to transmit idle (eighth rate) frames for a user defined time if there are no packets to send. This results in wasted bandwidth, especially in high-speed data systems, because the traffic channel capacity is reserved for this user and cannot be utilized by any other user. Thus, an access scheme that allows two or more users to share traffic channels, and allows the network to control the packet delay is needed.
In connection-less packet data transmission in mobile communication systems, discrete packets are transmitted on a packet-basis, that is, no dedicated end-to-end connection is set up. Sharing of traffic channels in connection-less packet data transmission has generally been supported via random access or random reservation media access control (MAC) protocols. Under these MAC protocols, multiple mobile stations compete for traffic channels in order to receive packet data service from a base station (BS). The number of available traffic channels for packet service within a cell is defined by the BS.
Random reservation protocols are generally believed to offer high channel utilization. However, in a CDMA based system that is approaching the system""s maximum capacity, random access attempts are more likely to be unsuccessful due to higher interference levels. Thus, as a cell approaches its maximum capacity of available traffic channels, multiple MSs compete for fewer and fewer available traffic channels. This normally leads to even further access attempts by the MSs. Often, the random attempts themselves increase the communications load in the cell and reduce traffic channel capacity.
It is a first object and advantage of this invention to provide an efficient method for transmitting packet data over a cellular communication network that overcomes the foregoing and other problems.
Another object and advantage of this invention is to maintain scheduled traffic channel sharing among multiple packet data users who are attached to a cell of a mobile communication network.
Another object and advantage of this invention is to provide the base station of a cellular communication network with control over which mobile station(s) may attempt access to the system during a specified period of time.
Another object and advantage of this invention is to provide the base station of a cellular communication network the ability to preemptively control priority and duration of mobile station access by employing a scheduling method which considers one or more parameters including priority access service, quality of service, and a maximum number of bytes per transmission.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention. More particularly, this invention is directed to solving the problem of inefficient packet data transmission in CDMA based mobile communication systems.
Scheduled packet access, as taught by this invention, leads to more stable load conditions, offers higher channel utilization, and enables determination and control of a maximum delay in packet data transmission experienced by users. According to the teachings of this invention, the BS divides access to the traffic channel(s) allocated for packet data services into time slots. The right to access a given traffic channel for a specific time slot is referred to as a packet token, hereinafter simply a token. One or more token(s) are allocated to the MSs from time slot to time slot, in a pre-defined manner. The token allocation schedule is determined at the BS, which may employ a variety of schemes to determine how the token is assigned. This pre-defined, scheduled token allocation is distinguishable over conventional random access and random reservation protocols. Token allocation, as taught by this invention, is accomplished by granting transmission access to an available traffic channel to each packet data MS in the cell according to specific, scheduled time intervals. This scheduled approach allows the BS to preemptively control traffic channel access among packet data MSs. Random access protocols, on the other hand, allow access to an available traffic channel as a packet data MS requests it. Thus, the conventional random protocols employ a first-come-first-serve method of traffic channel access assignment.
Allocating the right to access an available traffic channel may not always result in a transmission from the packet data MS. If, at the end of a time slot, the BS has not received a valid transmission from the MS(s) who are allocated the token(s), either because the MS has nothing to send or an access preamble did not reach the BS successfully, then the BS allocates the token to the next MS(s) in the cell. In this way, the MSs share the packet traffic channels in a time-multiplexed fashion, and the BS schedules and controls the allocation of the traffic channels at all times. This technique eliminates the problems associated with conventional MS random access attempts to gain a traffic channel for transmitting a packet.
This invention preferably employs a technique in which a BS transmits at least one dedicated MAC channel on the forward link. When transmitting the MAC channel messages to a specific MS the BS may use a permuted electronic serial number (ESN) of the MS as a long code mask, and when broadcasting MAC channel messages to all MSs within a cell the BS preferably uses a public long code mask. The MAC channel messages convey packet data traffic channel information and status information on the allocation of the token(s) to packet mobile stations in the cell. The MAC channel messages are updated to reflect the latest token usage. By decoding the latest MAC channel message a MS evaluates who has the token(s) for the next time slot, and is able to predict when it should next be allocated the token from the BS. The MS holding a token may attempt to access the traffic channel if it has data to send. The MS preferably uses its own private long code for reverse link transmission, as the BS expects a preamble signal from the MS(s) currently holding a token. Every packet traffic channel has a pre-defined Walsh code associated on the forward link to which the MS listens to determine whether its access was successful. After an access acknowledgement is received from the BS, the MS terminates the preamble and starts to transmit its packet data. At this point, the MS may negotiate the traffic channel data rate with the BS. The initial traffic channel data rate is pre-defined by a Service Option and may be as low as a predefined low speed data service, such as 9.6 kpbs.
Once occupied, the traffic channel is preferably allocated to that MS until the end of the packet. The maximum time a MS is allowed to occupy a channel is predefined by the network, so that the BS can predict worst case channel usage. The BS may employ various techniques to ensure efficient channel usage. For example, when assigning a token, the BS may define a maximum number of bytes that a MS may transmit. If this maximum number of bytes per transmission is exceeded, the BS has the option to terminate a packet transmission by sending a transmission stop bit on the associated forward link traffic channel. If the transmission is stopped, the MS relinquishes the token at the next time slot and rejoins the pool of MSs awaiting the next available token. Similarly, if the transmission of packet data completes the MS yields the token at the next time slot. These techniques for monitoring transmissions may be based on equal sharing between MSs, or allow for different priorities of transmission which depend on the quality of service selected.
In accordance with the present invention, the BS allows the MS to turn off its receiver and save power during the periods when the MS does not hold the token. If the MS does shut down its processing between tokens the BS informs the MS, before the MS shuts down, when it should start its processing again, i.e. when the MS will be allocated the token again. This notification by the BS before the MS shuts down is possible as the token is assigned in advance. Thus, a xe2x80x9cdynamic slotted modexe2x80x9d operation is provided, the operation being dynamic in that the position of the slot need not be the same in every cycle. The slot position is a function of how many active MSs are on the channel, and on how much data each MS can transmit. Additionally, the number of slots is a function of the number of available channels at the BS.
The BS may transmit packets destined for a MS, if any, or fill data, on the forward link at the same time that the MS is allocated a token and is transmitting packets on the reverse link. This allows the BS to send power control information to the MS.